Pete, to my rather certain knowledge all Airstreams, from 1968 on at least, maybe before, have an internal water pressure regulator set at about 42psi. The only external regulator needed should be
adjustable and have a pressure gauge. It would be needed only if the "shore side" (city) water pressure were abnormally high which IMO would be above 80 psi. The internal regulator is situated very close to the incoming water supply and, of course, is in that line.

Hope this clarifies and does not confuse.

Donald WBCCI #2875